Apparatus and method for handling liquefied gas mixtures



G. R. BENZ 2,993,343

APPARATUS AND METHOD FOR HANDLING LIQUEFIED GAS MIXTURES July 25, 1961 Filed June 28, 1957 INVENTOR. GEORGE R. BENZ BY 2 i p ATTORNEYS United States Patent 2,993,343 APPARATUS AND METHOD FOR HANDLING LIQUEFIED GAS MIXTURES George R. Benz, Bartlesville, 0kla., assignor to Phillips Petroleum Company, a corporation of Delaware Filed June 28, 1957, Ser. No. 668,726 6 Claims. (Cl. 62- 48) This invention is concerned with apparatus and method for handling liquefied gas mixtures. In one aspect, there is provided a method and apparatus for handling liquefied gas mixtures having components of widely different boiling points, so that upon withdrawal of a portion of the mixture from a storage tank, the remaining stored liquid can be maintained at a fairly constant composition during the operation of the apparatus. In particular, the invention relates to the utilization of a mixture of liquefied gaseous hydrocarbons, components of which diifer in molecular weight by two carbon atoms, such as a mixture of ethane and normal butane.

Over the past decade the utilization of liquefied petroleum gas (L.P.G.) has grown in industrial importance, particularly as a fuel. Usually, the particular gas has been propane or butane, and methods have been devised for the utilization of a mixture of propane and butane, whereby, upon withdrawal of a portion of the fuel from the tank during usage, the composition in the tank does not undergo an undesirably large change. However, the boiling points of C hydrocarbons and C hydrocarbons, such as ethane and normal butane, respectively, are so widely different that there is a need for a new method of handling such a mixture allowing closer control of the composition when withdrawing portions thereof from a storage tank.

It is, therefore, an object of the invention to provide an apparatus and a method for handlingof liquefied gaseous hydrocarbon mixtures wherein components of the mixture contain a dilference of 2 carbon atoms in the molecule.

It is a further object of the invention to provide an automatic method and apparatus for withdrawal of vapor only from a storage tank containing a mixture of such liquefied hydrocarbons during periods of low demand and for withdrawing both liquid and vapor from said storage tank during periods of higher demand.

The invention is applicable to a mixture of hydrocarbons, one component of which contains 1 or 2 carbon atoms per molecule and another component of which contains 3 or 4 carbon atoms, respectively, per molecule.

The problem solved by the invention will be apparent from a consideration of the following facts. When storing a mixture of, say, 30 weight percent ethane and 70 weight percent normal butane in a storage tank, withdrawals of liquid only from the storage tank will increase the volume in the vapor space, and, since ethane is so much more volatile than butane, vaporization of ethane from the liquid in the tank to adjust forthe increased vapor volume in the tank will appreciably change the concentration of the liquid. On the other hand, should withdrawal be made from the vapor space alone, such withdrawal will be a withdrawal mainly of ethane, thus increasing the overall concentration of butane in the storage tank. Thus, withdrawal from the liquid will increase the overall concentration of ethane in the storage tank while the withdrawal fromthe vapor will increase the overall concentration of butane in the storage tank.

Therefore, according to my invention, I have conceived and provided a method for controlling this concentration by with drawing both from the liquid and the vapor at the same time so as to maintain a relatively constant overall concentration in the storage tank. However, during periods of relatively very low demand there is also ice the problem that it is impractical to withdraw liquid for such low demands as maintaining a pilot light, etc.; and, during such periods, according to my invention, I withdraw from the vapor space only.

Therefore, according to my invention, there is provided an apparatus comprising a storage tank for storage of a mixture of liquefied gases, a first conduit communicating with the bottom portion of said tank for withdrawal of liquid, a second conduit communicating with the top portion of said tank for withdrawal of vapor, the other end of said second conduit commuicating with a point of delivery of the vapors contained in said line during its operation, said second conduit communicating at a point intermediate its ends with said first conduit, a valve in said first conduit operatively connected to a pressure controller responsive to the pressure in said first conduit on the downstream side of said first valve, a second valve in said second conduit intermediate said tank and said intermediate point in said second conduit responsive to a pressure controller sensitive to the pressure in said second conduit downstream from said second control valve, said second valve opening in its operation in response to a predetermined minimum pressure and said first valve opening in response to a second predetermined minimum pressure slightly lower ban the said first predetermined minimum pressure. In the conduit between said tank and said first control valve there is located means for providing that fluid flowing in said first conduit through said first valve is maintained in vaporous condition.

The accompanying drawing is a schematic illustration of an embodiment of the process and apparatus of this invention.

Conduit 1 is for the introduction of the liquefied gaseous hydrocarbon mixture through valve 2 into high pressure storage tank 3. Conduit 4 communicates the lower portion of tank 3 with vaporizer 5, and contains pressure reduction valve 6. This valve is suitably a spring loaded valve which reduces the pressure from tank pressure to a considerably lower substantially constant pressure. This constant pressure is low enough so that, under the temperature conditions maintained in vaporizer 5, there is assured complete vaporization in vaporizer 5. An amount of superheat in vaporizer 5 is also often desirable in order to avoid any possible difiiculties when the pressure is further reduced in passing through valve 15. Preferably, the temperature in vaporizer 5 is maintained substantially constant and not above F., although higher temperatures, as well as lower temperatures, are applicable. Steam coil 7 is provided to furnish heat of vaporization to maintain the desired condition in vaporizer 5. It is connected to steam inlet conduit 8 containing valve 9. Valve 9 can be hand regulated to maintain a reasonably constant temperature in vaporizer 5 or it can be a control valve responsive to temperature sensitive means (not shown) within vaporizer 5. Vapor withdrawal line 10 communicates tank 3 with manifold 11, and contains valve 12 which is responsive to pressure controller 13. Pressure controller 13 is sensitive to the pressure in line 10 downstream from valve 12. The conduit 14 communicates the vapor space of vaporizer 5 with line 16 at a point downstream from control valve 12, and contains control valve 15 which is responsive to pressure controller 16. Pressure controller 16 is sensitive to the pressure in line 14. As will be understood, it is not important whether pressure controller 13 is sensitive to the pressure in line 10 or in line 14. The same is true of pressure controller 16. Manifold 11 supplies a plurality of points of consumption of the gaseous mixture; for instance, one end of the manifold can lead to a pilot light of a furnace, while another branch can supply gas to a burner of the same furnace.

Valve 12 is sized so that, when completely open it will pass only a very small fratcion of the amount of gas which valve 15 can supply when completely open. Pressure controller -13 is-set' so that valve 12 will open when a predetermined downstream pressure considerably lower than .the pressure existing upstream of valve 12 is reached,

while pressure controller 16 is set so that valve 15 will initially open at a slightly lower pressure than valve 12.

In the operation of this apparatus, tank 3 is first partially filled by a gaseous hydrocarbon mixture as hereinbefore defined, for instance, a mixture of ethane and butane. Normally, such additions are only periodic and valve 2 is closed after the tank is filled. During withdrawal of gaseous mixture through manifold 11 to points of consumption, if the flow is large enough the predetermined low pressure at which pressure controller 16 is set will cause valve 15 to open, and, since this pressure is lower than the predetermined low pressure at which controller 13 is set, valve 12 will also be open. Thus, vapors will be withdrawn from tank 3 through line 10 to manifold 11 in a smal volume, while liquid will be withdrawn through line 4 in a relatively larger amount, its pressure reduced and at least partially vaporized while passing through valve 6 into vaporizer where it is heated and all converted to vapor form. These vapors then pass through valve 15 in line 14 to line 10 and manifold 11 in much larger volume than the vapors passing through valve 12. Withdrawal of both liquid and vapors from tank 3 maintains the overall composition in tank 3 more constant than if withdrawals were made from the vapor space alone or the liquid alone. When the consumption of gas through manifold 11 is greatly reduced so that the pressure in line 14 is raised above the predetermined low pressure for which controller 16 is set but below the predetermined low pressure for which pressure controller 13 is set, valve will shut OE and a relatively very small flow of vapors will continue through valve 12. When consumption of the gaseous mixture through manifold 11 is entirely terminated, the pressure in line 10 to which controller 13 is responsive will rise above the predetermined low pressure for which that controller is set, and valve 12 will, therefore, close.

In an example of an embodiment of the invention, tank 3 is a 10,000 cubic foot tank and at the moment contains 500 cubic feet of liquid which is weight percent ethane and 70 Weight percent normal butane. The temperature of the liquid existing in the tank is 80 F., and the pressure therein is 255 p.s.i.a., while the vapor composition is 78.3 weight percent ethane and 21.7 weight percent butane. Thus, it can be seen that there is considerable fractionation in the tank because of the widespread difierence in volatility of the two hydrocarbons. The upper branch of manifold 11 leads to a pilot light of a furnace, while the lower branch leads to the furnace burner. The pilot is turned on by opening the valve associated therewith. The pressure in line 11 downstream of valve 12 falls to just below its preset value of 1 p.s.i.g., valve 12 opens, and gas flows therethrough to the pilot which is lighted. The pilot consumes 5 s.c.f. of gas per hour (60 F. and 14.7 p.s.i.a.). After five minutes, a valve associated with the furnace burner is opened and the pressure in line 10 and in line 14- downstream of the respective valves falls to below the preset value of pressure controller 16 of three-fourths p.s.i.g., valve 15 opens, and the burner is lighted by the pilot. The burner consumes 500 s.c.f. of gas per hour. During the time while the burner is on, valve 12 passes 10 s.c.f. per hour of thegaseous mixture from the vapor space, the maximum amount which valve 12 will pass because of its size. The liquid flowing through line 4 has its pressure reduced to'a constant value of 150 p.s.i.a. on the downstream side of valve 6, while the amount of steam admitted to coil 7 is controlled to maintain a temperature of approximately 80 F. inthe-vapor space of vaporizer 5. After one hour of operation, furnace burner 11 is shut-off, the pressure rises in line 14 to above three-fourths p.s.i.g. and valve 15 automaticaly closes, but the pressure stays below 1 p.s.i.g. and therefore, valve 12 stays open and supplies 5 s.c.f per hour of the gaseous mixture to the pilot.

Other examples of other liquefied gas mixtures to which the invention is applicable include mixtures containing normal butane and ethylene, mixtures containing isobutane and ethane, mixtures containing isobutane and ethylene, and mixtures containing propane and methane.

The ratio of the hydrocarbon in the liquefied gaseous hydrocarbon mixture of the invention containing 1 or 2 carbon atoms to the hydrocarbon containing two more carbon atoms per molecule will most often lie in the range from 2:10 to 5:10 by weight although ratios outside this range are applicable.

The pressures at which control valves 12 and 15 are set to operate can vary considerably and depend to a great extent upon the pressures existing at the final utilization of the gaseous mixture, the distance of the final utilization from the valve, and other factors, as will be evident to those skilled in the art. For instance, valve 12 could be set to open at 25 pounds while valve 15 could be set to open at 20 pounds when a high pressure utilization of the gaseous mixture is effected, as in a high pressure burner.

As will be evident to those skilled in the art, various modifications of this invention can be made or followed in'the light of the foregoing disclosure and discussion without departing from the spirit or scope of the disclosure or from the scope of the claims.

I claim:

1. A high pressure tank for storage of a liquefied gaseous hydrocarbon mixture; a first outlet conduit connected to the upper portion of said tank for withdrawal of vapor from said tank; a first normally closed control valve in said first conduit responsive to a first predetermined pressure in said first conduit downstream from said first valve to open said first valve; a second outlet conduit connected to the lower portion of said tank for conveying liquefied gas from the lower portion of said tank; vaporizing means in said second conduit converting said liquefied gas to a gaseous state; downstream from said vaporizing means, a second normally closed control valve in said second conduit responsive to a second predetermined pressure lower than said first predetermined pressure in said second conduit downstream from said second control valve to open said second valve, said first and second conduits merging into a common line downstream from said first and second control valves, respectively.

2. Apparatus of claim 1 wherein said vaporizingmeans comprises an indirect steam heater in a vessel in said second conduit.

3. Apparatus of claim 2 wherein said vaporizing means includes a pressure reducing valve in said second conduit between said tank and said vessel.

4. A method for handling and supplying liquefied gaseous hydrocarbons which comprises maintaining a confined body of liquefied gas hydrocarbons comprising a mixture of two hydrocarbons, the first of which contains from one to two carbon atoms per molecule and the second of which contains two more carbon atoms per molecule than the first, in a first enclosed zone under conditions such that vapors of said hydrocarbons exist in said first zone above said liquefied hydrocarbons; initially withdrawing vapors from said first zone at a first predetermined rate and passing same to a zone of consumption; and thereafter withdrawing liqiuid from said zone at a second predetermined rate greater than said first predetermined rate on a standard cubic feet of gas consumed basis, vaporizing said liquid, and passing said vaporized liquid to said zone of consumption While continuing to withdrawsaidvapors, whereby the fractionation of components of said confined body of liquefied gas hydrocarbons during said withdrawal is lessened.

5. A process of claim 4 wherein, during periods of low consumption said withdrawing of liquid is terminated and said withdrawal of vapors is continued at no higher rate than said lower rate.

6. A roms of claim 4- wherein the withdrawn vapors and the vaporized liquid are combined and thereafter passed to a point of consumption.

References Cited in the file of this patent UNITED STATES PATENTS Melsheimer Apr. 4, 1944 D11 Rant Sept. 15, 1953 Thompson Apr. 12, 1955 Northgraves Ian. 10, 1956 Thompson May 29, 1956 Chaisser et a1. Oct. 14, 1958 

